Grappling AoO for channelling Duskblade? Ouch!

EDIT: The second text was badly confusing since I posted it before the coffee kicked in. Here's a better description:
Last night my Duskblade managed to get himself entangled and later grappled by an Assassin Vine.
Dodging the first touch attack he considered the threat, a weird plant he'd never encounterd before. "It's just a plant," he thought, "I'll hit it with a scorching ray." Casting defensively to avoid an AoO, he blasted it with fiery death. Nothing happened!
"Electricity then," he thought, fending off the constricting tendrils a second time (still luckily avoiding a grapple). Again casting defensively, he fired off a Seeking Ray. Nothing!
"Melee it is," he said to no-one in particular.
The next round, the thing got a touch attack to land and with its Improved Grapple, I was immediately grappled. It went for extra constriction damage but the opposed check held this off. Desperately, he wiggled his dagger free. Avoiding further constriction the next round, he channelled a Blade of Blood spell and stabbed the blighter, right between the branches.

"Aha!" - crowed the DM - "you weren't casting defensively. Attack of Opportunity matey!"
"No!" I cried, "casting a spell in this way does not provoke an attack of opportunity - see, Arcane Channelling, I knew it said that and that's why I never called it."
"Ah yes," he replied, "but you're in a grapple and the spell has a somatic component so that doesn't count. Wham! Ten points of thorny scrape your face off damage, I'm afraid."

He's open to arguments for the next time though - was he right or mistaken?


The previous text is confusing and should not be read, except to understand how confusing my first explanation was to the following post, to whom I am very grateful!

Last night my Duskblade managed to get himself entangled and then grappled by an Assassin Vine.
"A plant," he thought, "I'll hit it with a scorching ray." Casting defensively to avoid an AoO, he blasted it with fiery death. Nothing happened!
"Electricity then," he thought, fending off the constricting tendrils a second time. Again casting defensively, he fired off a Seeking Ray. Nothing!
"Melee it is," he said to no-one in particular.
Fending off the constriction again, he wiggled his dagger free. Channelling a Blade of Blood spell he stabbed the blighter, right between the branches.

"Aha!" - crowed the DM - "you weren't casting defensively. Attack of Opportunity matey!"
"No!" I cried, "casting a spell in this way does not provoke an attack of opportunity - see, Arcane Channelling, I knew it said that and that's why I never called it."
"Ah yes," he replied, "but you're in a grapple and the spell has a somatic component so that doesn't count. Wham! Ten points of thorny scrape your face off damage, I'm afraid."

There's a lot wrong there, to my mind. When grappled, you can't cast spells with somatic components at all. That's why Teleport and Dimension Door are V only. The Duskblade Arcane Channeling ability states that you cast the spell and channel it through your weapon, so you couldn't cast BoB in the first place. If the class just channeled the arcane energy through the weapon, then you'd probably be able to, but the fact that the ability says you cast a spell means you're out of luck there. Speaking of, you do have either the Somatic Weaponry feat from Complete Mage or a free hand, right? Otherwise, by RAW, you can't cast spells with S components in the first place.

Also, why were you trying to cast defensively? While you were grappled, the vine was grappled too (unless it used the Improved Grab ability's option to take a -20 on the roll to not be so). Thus, neither of you threaten any spaces, so you wouldn't have to worry about AoO from the vine. In any event, you would have to make DC 20+spell level Concentration checks to cast while grappled. Grappling is teh suck for spellcasters.

The fact that you failed to declare that you were casting Blade of Blood defensively while in the grapple shouldn't have opened you up to an Attack of Opportunity, but neither should you have been able to cast any of the spells you did by virtue of being grappled.

Aha - makes a lot more sense. The first two rays I fired off, I wasn't in the grapple - sorry. The first two rounds I managed to avoid his touch attacks (it missed, basically) and tried to blast it away. Hence the casting defensively for the rays while in the threat range.

Once I was grappled I grabbed my dagger to try and stick it. My left hand was empty, though whether it was free to move or not is probably open to interpretation depending on how it had me grappled. I imagined it as winding round my body and going for the throat, with me frantically sawing, and described it as such. The DM found it reasonable (giving me a chance, I see now). Normally I use a longsword and buckler, leaving a hand free for casting.

So basically, I shouldn't have been able to cast in the first place, and if I had cast a non-somatic spell, I would have not had an AoO from the plant grappling me since it was . . grappling me. Got it! I also believe the DM had pumped it up a bit, since it was grappling multiple opponents and had better resistances than in the MM. Blimey - it's never simple!

I shouldn't have added the whole story halfway through posting, it made the situation description confusing. Thanks for the clarification and apologies for my poor explanation. I'll edit the post for the benefit of any future posters!